1. Field of the Invention
The present invention relates to an apparatus for making self-piercing nuts which each can pierce a metal plate to punch an opening therethrough, wherein a peripheral edge portion thereof is deformed to fit in a groove of the nut thus secured to the metal plate.
2. Description of the Prior Art
A variety of the self-piercing nuts, taking place of the weldable nuts or the clinch nuts, are widely used for example in manufacture of automobile vehicles which have many parts screwed with bolts to metal plates. Any self-piercing nut must withstand a torque so as not to rotate along with the bolt, and must clinch strongly to resist evulsion from the metal plate. Recently, self-piercing nuts of novel and high stress types have been demanded to meet a requirement for a higher fastening force.
Generally, each self-piercing nut has a cylindrical pilot portion having a female thread as well as an end surface serving as a punch. The pilot portion formed centrally of a nut body is surrounded by a rim extending along a periphery of the nut body and protruding therefrom in the same direction as the pilot portion. Therefore, an annular groove is provided between the pilot portion and the rim. In one of the high stress type, the nut body is of a tetragonal or hexagonal shape so that the annular groove has a non-circular configuration to improve the torque resistance. In another high stress type, at least one of the rim and the peripheral wall of pilot portion facing the rim is slanted relative to the axis of the nut body. The annular groove defined by such a slanted member or between the slanted members has a width increasing from its opening towards its bottom, thus being dovetailed in cross section to improve its clinching property.
FIG. 5 illustrates a self-piercing nut 1 of the high stress type secured to a metal plate 10. This self-piercing nut 1 comprises a pilot portion 3 which has a threaded bore 2 formed centrally of a nut body. A rim 4 extending along a periphery of the nut body and protruding therefrom surrounds the pilot portion, and an annular groove 5 is provided between the rim and the pilot portion. Both the side surfaces 6 and 7 of pilot portion 3 and rim 4 are slanted relative to the axis of the nut body, so that the annular groove 5 is dovetailed in cross section. In use of this self-piercing nut, a caulking die 8 cooperates with an end surface of the pilot portion 3 to punch an opening through the metal plate 10. A peripheral edge portion 10a of the punched opening will be deformed and forced to fit in the annular groove 5 of the nut 1 thus secured to the metal plate 10.
As seen in FIG. 5, an outer circular edge 3a of the pilot portion 3 (is an upper circular edge of one of the peripheral walls 6 which face one another to define the annular groove 5) serve as a cutting blade which cooperates with a caulking die 8 to punch the opening through the metal plate 10. The diameter D.sub.1 of the outer circular edge 3a, that is an inner diameter of the annular groove 5, must be highly accurate in dimension. The other peripheral wall 7 defining the annular groove has an inner circular edge 7a. This edge 7a exerts a clinching force to a peripheral edge portion 10a of the opening formed through the metal plate 10, after said edge portion 10a is deformed and forced by the caulking die 8 into the annular groove 5. The diameter D.sub.2 of said inner circular edge 7a, that is an outer diameter of the annular groove 5, also must be highly precise in dimension for invariable and reliable clinching force.